The present invention relates to multichannel communications systems, and more particularly to a compatible AM stereo system employing a modulated signal having differently phased carriers, where the phase angle between the carriers is dynamically varied.
Interest in transmitting stereophonic information over the AM frequency band has existed for more than 50 years, nearly as long as commercial AM broadcasting, itself, has existed. During this time, many different schemes have been suggested for communicating the stereophonically-related audio signals from the broadcasting station to the radio receivers. None of these schemes, however, has met with general approval by the broadcasting community since none has demonstrated a clear superiority over the others.
A number of criteria are commonly used in comparing the performance of the various systems. Generally stated, these criteria include the quality of stereophonic reproduction in stereophonic receivers and the compatibility of the transmitted stereo signal for reception by currently available (monophonic) AM receivers. In addition, it is desired that the stereophonic signals transmitted should not occupy any greater RF bandwidth than that presently allocated for monophonic AM transmission.
More specifically, the stereophonic performance of an acceptable AM stereo system should be such that, upon reception, the signal-to-noise ratio is as great as possible. In any event, it should not be significantly degraded as compared to reception obtainable with current monophonic systems. Also, the distortion introduced by the transmission and reception of the stereo signal should be minimal. Finally, the separation between the stereophonically related signals (usually referred to as the left (L) and right (R) signals) should be as great as possible.
With respect to mono-compatibility, any acceptable AM stereo system must be fully compatible with monophonic receivers currently available on the market. In other words, the detection of the composite stereo signal with the monophonic envelope detectors and product detectors currently in use should produce a signal corresponding to the sum (L+R) of the two stereophonically related signals, without noticeable distortion. Additionally, the loss in the loudness of the received signal in monophonic receivers due to the stereophonic nature of the broadcast signal should be as low as possible.
In a system proposed by Harris Corporation several differently-phased carriers are separately modulated and then added together to produce the composite modulated signal for transmission. One of the carrier signals is modulated by the L (left) audio signal, whereas the other carrier is modulated by the R (right) audio signal. In this system, referred to as the compatible phase modulation system (or CPM system), the angle between the two modulated carriers (referred to occasionally hereinafter as "modulated phase components") is set to a value of around 30.degree.. In another method of generating the same composite modulated signal, a conventional quadrature AM transmitter is used. A signal corresponding to the sum of L and R audio signals is used to modulate the in-phase channel, and a signal corresponding to the weighted difference between the L and R audio signals is used to modulate the quadrature-phase signal. The angle of 30.degree. between the L and R modulated phase components in the resulting composite modulated signal is established by appropriate weighting of the quadrature-phase modulating signal.
Signal-to-noise ratio (SNR) in stereophonic receivers for receiving this signal is dependent upon the phase angle employed, and would be greater at greater phase angles. It would therefore be desirable to utilize a phase angle which is greater than 30.degree. in order to improve SNR in stereophonic receivers. Unfortunately, to do so would increase distortion in conventional monophonic receivers to above acceptable levels.